Polyolefins such as polyethylene (PE), polypropylene (PP), etc., are lightweight and inexpensive and have excellent physical properties and processability. Due to such advantages, these polyolefins are used in a variety of fields, including food packages, clothes, containers for medical and cosmetic applications, automobile parts, communications and electrical device parts, engineering and construction materials, agricultural materials, medical devices, etc., and occupy a very important position as general-purpose resins. However, in recent years, demand for improved physical properties of polyolefins has been diversified as the applications thereof are extended. For example, polyolefins having excellent heat resistance or having a soft feel similar to that of soft polyvinyl chloride, which are absent in existing polyolefins, or highly functional polyolefins having good printability, coatability, adhesiveness, compatibility with other polar polymers, etc., are required.
In general, ethylene-based copolymers are non-polar materials which have no polar group in the molecule and, thus, exhibit low adhesiveness to highly polar materials such as metals or polar resins.
For this reason, when an ethylene-based copolymer is to be bonded to a highly polar material, the surface of the ethylene-based copolymer needs to be treated with flame, corona discharge, primer, etc., thereby causing a problem of complicated operation.
A metallocene compound refers to a group 4 transition metal compound having one or two cyclopentadienyl group(s) as (a) ligand(s). The metallocene compound can be used as a catalyst for olefin polymerization after activation with a methylaluminoxane or boron compound. Because the metallocene catalyst advantageously has uniform active sites, copolymerization is easy, a polymer prepared using the metallocene catalyst has a narrow molecular weight distribution, and the distribution of a second monomer is uniform. And, in the case of propylene polymerization, the steric structure of the polymer can be controlled according to the symmetry of the catalyst. In particular, whereas the use of the existing Ziegler-Natta catalyst enables the preparation of isotactic polypropylene only, the use of the metallocene catalyst enables the stereoregular preparation of various polypropylenes, including isotactic, syndiotactic, atactic as well as hemiisotactic polypropylenes. For example, a syndiotactic polypropylene synthesized using a metallocene catalyst has the characteristics of low crystallinity, suitable rigidity and hardness, good transparency and high impact resistance.
These metallocene catalysts are actively used for the production of LLDPE, VLDPE, EPM and EPDM, which are copolymers of ethylene and an α-olefin; cycloolefin copolymers (COCs) which are copolymers of ethylene and a cycloolefin, or an α-olefin and a cycloolefin; and copolymers of ethylene, an α-olefin and styrene. The catalysts used for the production of such polymer products are commonly required to have good activity and good reactivity with a second monomer and to be able to prepare a polymer wherein the distribution of the second monomer is uniform.
Meanwhile, because the metallocene catalysts are costly, compared to the existing Ziegler-Natta catalysts, they should have good activities to be economically valuable. In particular, one catalyst having good reactivity with the second monomer is advantageous in that it enables to produce a polymer with a high content of the second monomer even when the second monomer is used in small amounts.